S parameters can be tricky. Reflections can be tricky, too. IBIS-AMI models have many parameters that need to be optimized for each particular application. Questions of this nature can be handled with more depth and completeness via Mentor technical support. If you wish to engage Mentor technical support to fully resolve your questions please proceed to Mentor’s Support Center and open a service request on HyperLynx SI/PI. An experienced signal integrity engineer experienced with IBIS AMI models will contact you directly to help you more fully resolve these questions.